The present invention relates generally to a method of protecting semiconductor switches against destruction or damage caused by a short circuit. The present invention relates especially to a protection in semiconductor switches with a limited switching speed.
Semiconductor switches are commonly used in electrical control applications, because their electrical control can be easily arranged and they contain no moving, wear-prone parts. Thyristors and different kinds of transistors, such as bipolar transistors, FETs (field effect transistors) and IGBT transistors (isolated gate bipolar transistors) are commonly used as switching components.
FIG. 1 is a simplified illustration of a known stepless power control 11 for changing the amount of electrical power fed from an AC voltage source 12 to a resistive load 13. The power control of FIG. 1 is suitable for use as e.g. a dimmer of a mains voltage lighting fixture. The power control 11 comprises a field effect transistor unit, which in this case consists of two FETs 14 and 15 connected in series so that the drain of the first FET is connected to an AC current source 12 via a low-resistance current measurement resistor 16, the drain of the second FET is connected to the load 13 and the sources of FETs 14 and 15 are connected to each other. The gates of the FETs 14 and 15 are connected to control circuit 17 which regularly emits connecting pulses for connecting the FETs to conducting state for the desired time. The mean value of the electric power fed to the load 13 depends on the cycle ratio used in the circuit, i.e. the ratio of the duration of the conducting state of the FETs to the duration of the connecting cycle. The duration of the conducting state of the FET is called switch-on period.
If there is a short circuit in the load 13 or the resistance of the load otherwise considerably decreases from its normal value, the current flowing through a conducting FET is considerably higher than that of normal situation. In a FET, the part of electric power that is transformed into heat can during a short circuit be so large that the overheating during one switch-on period destroys or damages the FET so badly that it won't work correctly. To avoid such damage, there is a differential amplifier 18 in the circuit of FIG. 1 for measuring the voltage loss in the current measurement resistor 16 and for sending an output describing the value thereof to the control circuit 17. If the current being led to the load exceeds that of a normal situation, the control circuit 17 reacts to the signal emitted by the differential amplifier by not sending gate voltage pulses to the FETs, so that there will be no current through the FETs.
A previously known short circuit protection control requires a current measurement resistor rated for a relatively large throughput current. A resistive component connected in series causes a power loss the magnitude of which is proportional to the resistance of the component. The power loss can be minimized by using a current measurement resistor of very small resistance, but this in turn requires stricter tolerances for the measurement circuit measuring the voltage over the current measurement resistor. Strict tolerances increase manufacturing costs.
The purpose of the invention is to disclose a short circuit protection for a power control with small manufacturing costs and reliable and fast operation in case the semiconductor switch is switched to conducting state when there is voltage over the switch.
The aim of the invention is reached by measuring the gate voltage change of the FET acting as the switching element during switching operation and comparing it to the drain-source voltage measured over the FET and by either allowing or disabling the switching of the FET to conducting state on the basis of the result of the measurement.
A characterizing feature of a power control according to the invention comprising a field effect transistor is that it comprises means for measuring the change of the gate potential of the field effect transistor in relation to the voltage over the FET between the source and the drain and means for either allowing or disabling the switching of the FET to conducting state on the basis of the result of the measurement.
The present invention also relates to a method of protecting a power control from the negative effects of an excessive throughput current. A characterizing feature of the method according to the invention is that in the method the change of the FET gate voltage potential is measured in relation to the voltage over the field effect transistor between the source and the drain at the start of the gate control pulse of the field effect transistor and that the switching of the FET to conducting state is either allowed or disabled on the basis of the result of the gate voltage change measurement.
When the voltage pulse is led to the gate of the FET, the gate voltage increases in a normal situation first to a certain intermediate value, after which it is almost constant for a while due to the so-called Miller effect. It is only after this that the gate voltage rises to its peak value. The voltage between the drain and the source decreases intensely simultaneously with the increasing gate voltage and its temporary constant phase, if the electrical current through the FET from its drain to its source stays within normal values. In case of a short circuit the current increases intensely, there is no noticeable decrease in the voltage between the drain and the source and there is no Miller effect. In the short circuit protection according to the invention the gate voltage of the FET is measured and compared with a reference voltage higher than the value where the increase of the gate voltage temporarily stops due to Miller effect. If the gate voltage exceeds the reference voltage when the voltage between the drain and the source is high, there is a short circuit. In this case, gate voltage input to the FET will be cut.
The present invention is especially suitable for use in applications in which it is desirable to switch the FET controlling the alternating voltage throughput to conducting state in other phases than during the zero point of the alternating voltage. The circuit according to the invention can be manufactured using inexpensive low voltage components.